[Update: it looks like they're all gone. Bookmark Club Nintendo and check back periodically however, as they tend to announce new shipments.]
The cradle itself is a simple charging solution that allows you to place your 3DS XL into the unit, with a hookup in the back for an AC adapter. Blue, red, and white are already sold out, so you'll have to deal with yellow and green.
3DS XL Charging Cradle [Club Nintendo] (Thanks Sarah!)